The LLY Duramax diesel engine of the 2004.5 to 2005 GM HD trucks was relatively short-lived, but it was notable for improving on its LB7 Duramax predecessor in several ways. New externally accessible injectors, a higher airflow turbocharger and (especially) revised common-rail injectors all made the LLY Duramax a much-improved engine. Even with these improvements, however, the engine has some nuances that can leave drivers wanting even more.

If you own a 2004.5 to 2005 GM HD and want more power, lower exhaust gas temperatures (EGTs) and better efficiency, these are the best LLY Duramax upgrades to make.

Less Restrictive Turbo Inlet Manifold

The stock turbo inlet manifold is one of the most frustrating components of the LLY Duramax diesel engine’s design. The part has a highly restrictive design, which forces the turbocharger to overwork if it’s going to supply a decent amount of extra air to the engine.

Replacing this piece with a high-flow turbo inlet manifold is a simple step that not only increases performance but also greatly reduces engine overheating — overheating is a particularly well-known problem of the engine because the LLY has the smallest radiator of any Duramax GM has made.

Larger Exhaust System

When you increase airflow entering the engine, it only makes sense to increase the airflow exiting the engine. Increase the exhaust to a 4-inch downpipe exhaust system, and you won’t have a pinch downstream of the engine.

Larger exhaust parts are widely available for the LLY Duramax, even though it was made for just a couple of years. Most compatible exhausts are also compatible with the LBY, LBZ and LMM Duramax engines, so there are a lot of these aftermarket parts.

Automatic Transmission

Once you put enough LLY Duramax upgrades in place, you also need to upgrade the transmission so that it can handle the increased power. A larger clutch will be needed if you run on a manual transmission. For all of the trucks that have an automatic, talk with a knowledgeable Duramax diesel technician about the changes to make during a rebuild or replacement.

Larger Downpipe

Anytime you pull the transmission, you should consider replacing the stock downpipe. GM’s stock downpipe is almost never as large as you’d want it to be, and this is an easy replacement to make while working on the transmission. Performing this LLY Duramax upgrade will further increase airflow and thereby improve power.

External Oil Cooler

If you make a lot of LLY Duramax upgrades and demand a lot of your truck, merely increasing airflow may not be enough to keep the engine cool.

There comes a point when you should add on an external oil cooler that will actively cool the engine’s oil. Depending on the specific one you choose, this could lower the engine’s operating temperature by 20 degrees or more.